A multitude of different methods are previously known for preventing the production of sand from wells. These methods are known under the generic term “sand control”. Sand control is used in wells that produce oil, gas and/or water, and it can also be installed in wells used for injection of water and/or gas. The purpose of installing some form or other of sand control in a well is to prevent sand and/or other particles from the formation from accompanying the flow of produced or injected fluid into the well itself. Most of the methods are characterised by the installation of some form of mechanical equipment in the actual borehole/well. Many of the methods also involve placing, by means of pumping specific fluids, specially adapted sand or particles of other material either in the space between the sand screen and the formation and/or into perforations within the formation.
There are also chemical methods for preventing sand production. In these methods, chemicals are used, which are pumped into the formation in order to increase formation strength and prevent failure. It is such formation failure that in turn leads to sand production. In addition, it is an alternative to coat specially adapted sand or particles of other material with chemicals that bind the particles together after they have been placed inside perforations. The actual placement of the chemicals, either with or without the addition of other particles, is carried out by pumping chemicals into the well with the aid of various fluids, such as carrier or displacement fluids.
A less used, but nonetheless efficient method of sand control is to perforate the well in formations that are consolidated and then fracture these formations. These cracks or fractures, which are made by pumping fluid into the well at a pressure that exceeds the fracturing pressure of the formation, grow within the formations, which are basically too weak to be produced without some form of sand control. This is called “Indirect Vertical Fracture Completion”. The fracture, which is formed by the fluids pumped into the well, is filled with sand or particles of another material such as to prevent the fractures from closing after the operation has been completed and the pump pressure has been removed. These fractures act as good flow paths for formation fluids from the unconsolidated formation, which can now flow into the well without taking with them particles from the formation.
The object of the earliest sand control methods was to place a “filter” in the well, i.e., a filter that prevented sand and particles from the formation from entering the well. Current technology is more sophisticated. Today, it is desirable that small and harmless particles should accompany the fluid flow from the reservoir into the well and up to the surface. The production of these small particles out of the well and also out of the sand control equipment itself results in a longer useful life for the well and the equipment. In addition, the fluids produced can have a higher volume flow because the pressure loss through the sand control equipment will be lower than if also the small particles were to be retained. The terminology used about today's more sophisticated methods is “propping the formation”, as opposed to the earlier filter analogy.
With the exception of the chemical methods, today's sand control methods and equipment are unsuitable for use in small lateral holes. Lateral holes should be understood as holes that extend from the main well at an angle of up to 90° compared with the angle of the motherbore. In some instances, the angle may be more than 90°. A common feature of all of today's sand control methods is that pipes of relatively large dimensions are installed in the well itself. These pipes, which have many different designs and configurations, are placed within the producing formation (or within the formation to be injected into). Such pipes are rigid structures of large diameter and large bending radius, and common to them all is that the dimensions are adapted to the diameter of the well itself, which means that they cannot be run out into or installed in small lateral holes.
The simplest form of sand control is to use a liner with milled-out slots in the longitudinal direction of the liner. In the industry these are known as a “slotted liner”. These special slotted liners are placed at a depth in the well where the producing formations are located, and they prevent sand and/or other particles from the formation from accompanying the liquid/gas flow, whilst the milled-out slots/sieves allow liquid and/or gas to flow into or out of the well. The size of the milled-out slots/sieves is adapted to the size of the sand and the particles found in the specific formation.
A common feature of the pieces of equipment referred to as screens is that they have a base pipe and that the actual mechanism for stopping the sand production lies as an integral part on the outside of the base pipe. This mechanism for stopping the sand and particle production is called a screen. Liquid and/or gas from the formation thus flows through the screen on the outside of the base pipe, then in through one or more holes in the base pipe and into the well itself. The screen on the outside of the base pipe is adapted to the size of the sand and particles in the formation such that the sand and particles are prevented from entering into the well itself. A very common sand screen structure that is used on the outside of the base pipe to stop sand production employs an equilateral metal wire with a trapezoid cross-section that is spiraled around the base pipe with a given space between each winding. Another method is to lay different layers of fine-meshed netting on the exterior of the base pipe. Some of the different layers may have the purpose of providing strength, whilst others are designed to stop sand production. On the outside again, that is to say, on the very exterior of screens of this type, there is a layer that is to protect the wire netting or mesh. This layer may, for example, have the form of a perforated pipe. There are also base pipes that are provided with ceramic rings on the outside. These are positioned with a spacing that is adapted to the size of the sand and the particles in the formation such that liquid and/or gas flows through and into the interior of the base pipe whilst the sand and particles are stopped. Outermost on screens of this type there may also be a perforated pipe that protects the ceramic rings themselves during both installation and production.
Lastly, there are screens which, on the outside of the base pipe, have a layer of specially adapted sand or particles of other material that are cemented together using chemicals of different types. Often a form of resin is used. On the exterior, there is in turn found yet another mechanical device which may have the form of an outer perforated pipe or of an equilateral metal wire with a trapezoid cross-section that is wound around the base pipe with a given space between each winding. The base pipe and the outer pipe form a chamber that locks in place the chemically cemented and specially adapted sand or particles. This solution is referred to as “pre-packed screens”.
Today, as a step towards increasing productivity, extending useful life and time at plateau production, the industry has started to construct small lateral holes out from the main well. These can be described as perforations. A main difference is that the perforations are much shorter in length than the lateral holes. Whilst the perforations can be up to two to three feet (0.6-0.9 meters) in length, lateral holes can be up to several hundred feet (up to several hundred meters) long. And whilst the perforations are made by means of directed blasting charges that are fired downhole, there are different methods for constructing the lateral holes, but a common feature of perforations and lateral holes is that they are at about 90 degrees to the well itself. Both perforations and lateral holes are formed after the actual well has been drilled, and usually also after the well has been completed. The well is thus drilled first, and then a liner and/or casing is installed in the well at the same level as the actual formation. This pipe is cemented in place by pumping cement out to the exterior of the pipe, i.e., between the pipe and the formation. Exterior to this cement lie the producing formations. To obtain contact between the formation and the well itself, the decision may be made to perforate the well and/or drill lateral holes. When lateral holes are to be drilled, tools are run into the well that first drill holes in the actual liner or casing. These holes are often around 2.5 cm in diameter (one inch). Then a tool is run into the well that is capable of drilling the actual lateral hole a long way out and away from the main well and into the formation. A frequently employed method is to use nozzles and liquids that flush away the formation, thereby making a lateral hole in the formation and at an ever-increasing distance from the well. The diameter and length of these lateral holes can vary according to need, drilling time used and the equipment employed. Today, equipment exists that can drill up to several lateral holes at one and the same depth in the well. With the exception of WO 2013/036133 A1, the equipment that has drilled the lateral hole is pulled out of the well after the operation has been completed. WO 2013/036133 A1 describes a system where the equipment is left in the lateral holes after drilling. During the production phase, liquid flows from the formation into the annulus between the drilling equipment and the formation and into the well itself. This method provides no sand control in the lateral holes themselves.
Lateral holes, too, may collapse or have limited production/volume flow owing to production of sand or particles from the formation. They therefore require a form of sand control such as to be able to justify the economics of the project through sand-free production of hydrocarbons and/or water, or injection of water and/or gas. An alternative may be to use the chemical methods that are available, but the systems that will fill the lateral hole with specially adapted sand or particles of another material are particularly unfavourable in such completions. This is because the methods result in a dramatic increase in the pressure drop for liquid and/or gas that is to flow through the lateral hole. This pressure drop can be modelled using Darcy's Law for linear flow through a porous material. Today's various sand screens are of a size, design and construction that prevent them from being installed in lateral holes. They are too large and rigid, and their design and construction also mean they cannot be scaled down to the dimensions required for lateral holes. Even if today's various sand screens could be scaled down, they would have a moment of resistance to bending that is too great to allow them to be installed in a lateral hole. A sand screen that is to be run into a lateral hole must be bendable at an angle of 90 degrees inside a liner or casing that has an internal diameter of 255 millimeters (10¾″) or less. In addition, this type of sand screen must have an external diameter that is smaller than the hole drilled in the liner or casing itself. Normally, this hole will be around 2.5 cm, but it may be larger or it may be smaller.